Spermatogenesis is a complex cellular process that is initiated with mitotic divisions of type A spermatogonial stem cells. The long-term objective of this application is to understand better the regulation of these initial steps of spermatogenesis, namely spermatogonial renewal, differentiation, or death. The applicant has succeeded in the isolation of highly pure populations of rat type A spermatogonia and demonstrated that these cell express the c-kit receptor. Stem cell factor (SCF) also phosphorylates this receptor in type A spermatogonia. Using this finding as a baseboard, the applicant now describes three Specific Aims to further the study of the spermatogonial life cycle. Specific Aim A will examine the effect of SCF, alone and in combination with leukemia inhibiting factor and bFGF, on spermatogonial survival, renewal and differentiation in vitro. Both the soluble and membrane bound forms of SCF will be studied. Specific Aim B will investigate the mechanisms of action of SCF in the spermatogonial population. Here, the principal objective is to characterize the signal transduction pathway(s) in type A cells which is activated by SCF. Also, the pathway(s) will be associated with spermatogonial survival, renewal and/or differentiation. Finally, Specific Aim C will test the hypothesis that telomerase is limited to spermatogonia and that the loss of this key enzyme is associated with the transition into meiosis. The applicant stresses the potential significance of work on isolated spermatogonia as it may relate to future studies for the transplant of stem cells to repopulate damaged or inoperative testes.